Statement of the Technical Field
The inventive arrangements relate to electronic circuit fault detection and more particularly to methods and systems for fault detection optimization.
Description of the Related Art
Nearly all circuit cards contain a substantial number of capacitors. These capacitors are most often used for decoupling, and are arranged in bulk groups connected between a primary power source and ground. This arrangement leads to a number of problems. For example, most bulk decoupling capacitors in such an arrangement are individually untestable for presence due to the fact that their individual value is a small percentage of the bulk capacitance. Accordingly, the presence or absence of one particular capacitor is difficult to detect.
Also, bulk decoupling capacitors make up a substantial percentage of the total component count of most designs. Thus, they make up a substantial percentage of the total number of likely defect locations. Further, defects associated with decoupling capacitors are among the most difficult and time-consuming to resolve. Common defects can include power-to-ground shorts, and degraded performance. Moreover, polarized capacitors cannot be electrically tested to ensure correct polarization.
Current technologies used to test for individual capacitors include automatic/manual optical test and electrical testing. Optical testing has the advantage of being sometimes able to detect a missing part. However, optical testing does not facilitate testing a component for value, and has at best only a marginal ability to locate either shorted or open pins.
Electrical testing is advantageous because it can detect the presence of a short in a network of capacitors. However, in most cases it does not facilitate detection of individual missing capacitor components. In order to understand why this is so, assume that a bank of twenty 1 uF at 10% tolerance capacitors needs to be tested, where the net capacitance of the entire bank=20 uF, +/−2 uF. Those skilled in the art will appreciate that up to two capacitors can be missing or tomb-stoned and the electrical test will still pass. In such a scenario, 18 of the capacitors can be considered tested, but which of the 18 are tested is not known. In fact, it is impossible to guarantee that any one of the 20 capacitors is actually tested, as any two can be missing.
Electrical testing also cannot detect open circuit pins on most individual capacitor components. The capacitors are high impedance devices and an open circuit measurement is normally expected when measuring between the two plates of a capacitor. Accordingly, open circuit pins are difficult to detect. Finally, electrical testing is not useful when trying to test a capacitor component discretely for shorted pins (i.e., a short circuit which electrically connects the plates of the capacitor). Shorted networks of capacitors can be detected. But, electrical testing does not facilitate identification of which capacitors are shorted because in many cases all of the capacitors in the network are connected in parallel. If one capacitor is shorted, all will appear to be shorted.
Troubleshooting power-to-ground shorts is complicated significantly by the typically large number of capacitors which reside on the affected nets. Current technologies used include optical inspection to locate visible shorts. However, an operator may need to check hundreds of components in order to locate a defect. In some scenarios, x-ray inspection of circuit boards is used to locate shorts underneath components. However, an operator may need to check hundreds of components in order to locate a defect. Other methods include Tone Ohm or micro-ohm meters to narrow likely short locations geographically on the board. However, operator experience is crucial in this method and an unskilled operator will not be able to use this equipment. Thermal imaging can also be used to locate hotspots which occur in the vicinity of a short circuit. Current is injected through the shorted nets, warming up the short while the entire assembly is monitored for thermal deviations. Extremely robust current injection points must be established, frequently necessitating soldering to the unit. Shorts internal to the printed wiring board can be ‘blown out’, making it impossible to determine where the short ‘was’. Reliability concerns typically prevent such a unit from being deliverable. An operator must be specially trained to properly use this method.